1. Field of the Invention
The present invention relates generally to memory, and more specifically to NVRAM.
2. Description of the Related Art
Most digital electronic systems have two generic memory requirements: an operating memory whose primary technical requirements are fast read and write and no wear out, and a storage memory (for data and/or program store) whose primary technical requirements are non-volatility and the ability to read many times without significant data degradation. The ideal memory would satisfy both these needs in a single device. In addition, the ideal memory will also achieve considerable cost/bit reductions as compared to existing memories.
Near ideal operating characteristics might be as follows:    Reads and writes <100 ns    Retains data after the power supply has been removed (non-volatile)    Operates at low power supply voltage    Has low active and standby currents    Does not wear out with read/write cycling    Has low cost/bit    Has simple read & write operation in a system
Wear out is the phenomena whereby a memory will fail to work after a certain number of read/write cycles. This number is usually between 100,000 to 10,000,000 read/write cycles. For a memory to have zero wear out, it would need the capability of cycling a minimum of 1015 read/write cycles.
While the present semiconductor memories that are currently in high volume commercial production today are far from meeting ideal specifications, they have achieved a large measure of commercial success because they work reasonably well at an acceptable cost. The major drawbacks from currently available non-volatile memories are as follows.                Have long write times that range from 5 us (microseconds) to 100 s of ms (milliseconds).            Wear out after a few hundred thousand read/write cycles.    Have a complex user operation that involves a separate erase operation.
However, the key parameter for most markets is cost/bit. Any solution that is more costly on a cost/bit basis will likely be relegated to a niche market.
Recently, there have been announcements of new technologies that are being investigated that could result in memories with closer to ideal operating characteristics. None of these new technologies have yet become a commercial reality. The leading contenders for the next generation memory are likely:    MRAM-MTJ (Magnetic RAM, Magnetic Tunnel Junction)    MRAM-GMR (Magnetic RAM, Giant Magneto Resistance)    OUM (Ovonics Unified Memory)    FeRAM-ITIC Ferroelectric RAM with 1 Transistor, 1 Capacitor    FeRAM-IT Single Transistor Ferroelectric FET    Ferroelectric Polymer    Chalcogenide Metal (Ag) Dendrite
There has been a fair amount of press given to each of the above technologies over the past few years. Unfortunately, none of the most promising technologies have achieved any measure of wide commercial success to date. The details on each of the above emerging memory technologies are as follows:
MRAM-MTJ is a very complex structure, and there appears to be problems scaling the cell size and the write current. While MRAM-MTJ has performance benefits, it is significantly more expensive than other solutions. There are some technical concerns about its scalability based upon the fundamental physics of magnetic materials, which will likely be very difficult to overcome. Unless MRAM can scale to small dimensions, it will be more expensive on a cost per bit, and not achieve wide acceptance in the market.
MRAM-GMR also is a structure that is physically large and so the same questions of cost/bit apply.
The OUM is a memory technology that has been around for 30+ years. In the past several years there has been a renewed interest in this technology, and presently the technology is at the stage of a feasibility design. However, there are still some technical difficulties associated with this technology due to the fact that the material is heated up to its melting point, during the write operation.
The FeRAM1T is a very new technology that solves the problem of fast writing, but has a significant cost penalty. This technology will likely never be less costly than standard Flash memory. It may have other advantages that make it suitable for certain niche applications.
The Ferroelectric polymer memory has slow performance and high temperature operating limitations at <85° C. However, it offers significantly lower cost per bit.
The Chalcognide Metal (Ag) Dendrite memory is in early stages of development, and its long-term success is unknown. Its electrical characteristics are such that it is unlikely to function as a non-volatile memory. Its high temperature (100° C.) characteristics are marginal as well as its data retention characteristics.
Table 1 summarizes each of the above emerging technologies.
TABLE 1Non-VolatileRAM OperationCostMRAMYesYesHighOUMYesYesLowFeRAM 1T1CYesYesMediumFeRAM 1TYesYesMediumPolymerYesNoVery LowMetal Dendrite?Yes?